The invention relates to a method and an apparatus for purifying a liquid by evaporation and with compression of the vapor.
It is quite well known to remove substances from a supplied starting liquid by allowing the liquid to evaporate and allowing the liquid to condense without the constituents to be separated from the starting liquid and collecting it. The evaporation can be promoted by heating the liquid. The non-evaporated liquid can be discharged or be recirculated. This last is generally attractive in processes where the starting liquid cannot be simply drained and is evaporated to a limited volume in which the unwanted constituents occur in a relatively high concentration. If the liquid after evaporation is hotter than the initial temperature prior to heating of the liquid as supplied, recirculation moreover enables a utilization of the remainder of heat supplied to the liquid, left after evaporation.
By compressing the captured vapor, it is heated and the dew point of the air and the vapor included therein is shifted to a higher temperature. By thereupon passing the air-with-vapor adjusted to an increased pressure through a heat exchanger through which, separately from the air and the vapor, also the recirculated liquid is passed, a heat flow to the recirculated liquid is created. This is particularly effective for recovering the energy introduced with compression, because a large portion of the vapor condenses in the heat exchanger, thereby releasing heat with which the recirculating liquid is heated. Optionally, in a second step, residual heat may be transferred to supplied liquid, so that it is preheated before being supplied to the recirculation circuit.
The evaporation of liquid can take place, for example, in the heat exchanger, which is simple and favorable to the efficiency, because the cooling action of the evaporation contributes to the heat flow through the heat exchanger.
A disadvantage of evaporation in the heat exchanger is that upon evaporation aerosols are formed which are entrained when capturing the vapor. Contained in the aerosols are constituents of the starting liquid that need to be separated in purifying the liquid. This means that the purification of the liquid halts at an incomplete level. Moreover, such entrained impurities entail pollution and hence diminishing performance of the heat exchanger.
An improvement can be achieved when for an effective separation of liquid and vapor evaporation is done in a cyclone. The liquid is then jetted into a cyclone, within which a portion of the liquid evaporates. From U.S. Pat. No. 6,365,005, for example, it is known to drive rotation of the liquid in the cyclone by introducing supplied liquid tangentially below the liquid level in the cyclone. For achieving a high efficiency and a high production capacity, however, an intensive evaporation is desirable. An intensive evaporation in the cyclone has been found still to entail an unwanted extent of pollution of the evaporated and condensed liquid.